Zipper cases. These are thin pillowcases that close with a zipper. You then put the normal pillowcase over the zippercase.
Why? Because it extends the life of your pillow. A normal pillowcase can slide while sleeping, etc… which can cause your face, hair, head, body, and all the oil it produces to get on the pillow itself. With the zippercase, you have a second line of defense to keep the pillow fresh.
Sure, you can clean the pillow, or run it through a gentle cycle in the wash, but it is way better to just avoid that. Let the double layer of protection do most of the work and absorb that body oil.
Change case 1 or 2 times weekly, change/wash zippercase weekly. Your pillow will stay fresh and clean and much more pleasant for longer, for a minimal amount of added work of putting that second case on.